1. Field of the Invention
This invention relates to a method for producing carbon-carbon composite materials obtained by impregnating pitch based carbonaceous fibers, as a reinforcement material, with a carbonaceous material such as pitch or the like and then heat treating the impregnated material at a high temperature in an inert gas atmosphere.
More specifically, it relates to a method for producing, high density, high strength, carbon-carbon composite materials useful in mechanical parts, electric or electronic parts by taking advantage of their high strength, thermal shock performance, chemical resistance hard wearing property and so forth.
2. Description of the Prior Art
It has been known that high density, high strength carbon-carbon composite materials can be produced by impregnating reinforcement material such as high strength, high modulus carbon fibers wound up to a desired shape or structures comprising carbon fibers as a principal material, with a carbonaceous material such as a thermosetting resin or a pitch which is a precursor of a carbon matrix, carbonizing the impregnated material in an inert gas atmosphere and if necessary graphitizing thus carbonized material.
However, fatal phenomena of cracking and detachment exist on the boundary between the reinforcement materials and the carbon matrix. As a result the mechanical strength of produced carbon-carbon composite materials was not sufficient.
On that account, it is necessary to repeat the process of impregnation/carbonization 5-6 times or more in order to get a high performance carbon-carbon composite material which has properties of high density, high strength and so forth.
In such production method, since impregnation and carbonization steps are repeated, it is necessary to expend much time and the process becomes more complicated. Moreover, there has been a further demerit that the product becomes much more expensive, for extension of working time of an electric furnace during the step of carbonization.
Further a conventional carbon-carbon composite material is weak in the adhesion on the boundary between carbon fibers as a reinforcement material and a carbon matrix. With the object of improving these drawbacks and increasing the adhesion, it has been done that the surfaces of carbon fibers are coated with various kinds of reagents.
However, there have been problems that the heat-treated product of the reagent becomes an impurity of resulting carbon-carbon composite material and reduces the purity as a carbon material. For that reason, the resistance to chemicals and heat is lowered. It is disclosed in Japanese laid open patent application No. Sho 52-52912 that in order to solve the above-mentioned problem, it is effective to use an organic polymer, which is the same material as raw organic fibers for producing carbon fibers, for a carbonaceous material as a precursor of carbon matrix; i.e. when polyacrylonitrile (PAN) based carbon fibers are used as a reinforcement material, the application of a polyacrylonitrile derived resin is effective, and when pitch based carbon fibers are used as a reinforcement material the application of pitch is effective.
According to a method of Japanese laid open patent application No. Sho 52-52912, a carbon matrix obtained by carbonizing a carbonaceous material shows nearly the same property with the carbon fibers as a reinforcement material. It provides some effectiveness to improve the adhesion between the reinforcement material and carbon matrix.
However, a carbonaceous material as a precursor of carbon matrix gives rise to volume shrinkage during the process of carbonization and graphitization. If rigid carbon fibers exist in the matrix precursor, crack and detachment are generated on the boundary between the carbon fibers and carbon matrix because the volume shrinkage of the matrix precursor differs from that of the carbon fiber during heat treatment at a high temperature in an inert gas atmosphere. Furthermore, the volume shrinkage of the matrix precursor does not always occur uniformly. Namely, since there is a large difference in orientation of carbon between that of carbon fibers as a reinforcement material and that of a carbonaceous material as a precursor of carbon matrix, there is a difference in each thermal property during the early stage of carbonization and generation of crack and detachment occurs, and so a secondary reinforcement treatment has been still necessary.
U.S. Pat. No. 3,814,642 discloses that the oxidized organic fiber or the oxidized and slightly carbonized fiber, which has the ether-bonded type oxygen content of from 3 to 15% by weight, is blended with an organic binding material and heat treated in order to increase the mechanical strength of the resulting carbon shaped article by carbon-oxygen-carbon (C--O--C) bonds.
But, since the organic fiber, especially pitch based fiber, including oxygen is very brittle and the processability for such as winding is very poor, the strength of the resulting carbon-carbon composite material is still insufficient.
Japan laid open patent application No. Sho 59-107913 discloses to use carbon fibers having essentially no functional group such as carboxyl group, carbonyl group, and hydroxy group. Since there is no effect of functional group in the phenolic resin shaped body, binding of carbon fibers and a phenolic resin is in the relatively weak state. And if these shaped bodies are subjected to carbonization treatment, the boundary between the phenolic resin derived carbon and carbon fibers, where binding is relatively weak as above-mentioned, is locally detached during the early stage of carbonization treatment.
On this account, local cracks are generated within shaped bodies during the carbonization treatment, but fatal detachments are not observed at the surface of the shaped bodies. However, since the problem of detachments caused by shrinkage is not essentially resolved, the strength of the resulting carbon-carbon composite material is still insufficient.
It is an object of the present invention to provide a carbon-carbon composite material superior in mechanical strength, resistance to heat and chemicals, hard wearing property by overcoming the above-mentioned drawbacks, namely by greatly reducing the fatal crack or detachment which occur on the boundary between the reinforcement material and the carbon matrix during the early stage of carbonization in the production process of carbon-carbon composite material and improving the adhesion of the boundary between the reinforcement material and the carbon matrix.
Another object of the present invention is to provide a process for producing carbon-carbon composite materials easily and at lower cost in commercial scale.